Near-surface electronic structure in strained Ni-ferrite films: An x-ray absorption spectroscopy study

Author:

Saha S.1ORCID,Knut R.2,Gupta A.3ORCID,Radu F.4ORCID,Luo C.45ORCID,Karis O.2ORCID,Arena D. A.6ORCID

Affiliation:

1. Department of Physics, Ashoka University 1 , Sonipat, Haryana 131029, India

2. Department of Physics and Astronomy, Uppsala University 2 , SE-75120 Uppsala, Sweden

3. Department of Chemistry and Biochemistry, The University of Alabama 3 , Tuscaloosa, Alabama 35487

4. Helmholtz-Zentrum Berlin für Materialien und Energie 4 , Albert-Einstein-Strasse 15, 12489 Berlin, Germany

5. Institute of Experimental Physics of Functional Spin Systems, Technical University of Munich 5 , James-Franck-Strasse 1, 85748 Garching b. München, Germany

6. Department of Physics, University of South Florida 6 , Tampa, Florida 33620

Abstract

We report on the x-ray absorption spectra (XAS) and x-ray magnetic circular dichroism (XMCD) of a series of NiFe2O4 (Ni ferrite) films grown on symmetry matched substrates and measured in two geometries: out-of-plane and near in-plane. The Ni ferrite films, grown by pulsed laser deposition, are epitaxial and the substrates used (ZnGa2O4, CoGa2O4, MgGa2O4, and MgAl2O4) introduce a systematic variation in the lattice mismatch between the substrate and the film. Modeling of the XAS and XMCD spectra, both measured with the surface sensitive total electron yield mode, indicates that the Ni2+ cations reside on the octahedrally coordinated lattice sites in the spinel structure. Analyses of the Fe XAS and XMCD spectra are consistent with Fe3+ cations occupying a subset of the octahedral and tetrahedral sites in the spinel oxide lattice with the addition of a small amount of Fe2+ located on octahedral sites. The Ni2+ orbital to spin moment ratio (μℓ/μs), derived from the application of XMCD sum rules, is enhanced for the substrates with a small lattice mismatch relative to NiFe2O4. The results suggest a path for increasing the orbital moment in NiFe2O4 by applying thin film growth techniques that can maintain a highly strained lattice for the NiFe2O4 film.

Funder

National Science Foundation

Swedish Fulbright Commission

USF Nexus Initiative

Science and Engineering Research Board

Axis Grant - Ashoka University

Carl Tryggers Stiftelse för Vetenskaplig Forskning

Vetenskapsrådet

Publisher

American Vacuum Society

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